Piezoelectric element



July 14, 1936. BLOOMENTHAL 2,047,252

PIEZOELECTRIC ELEMENT Filed June 9, 1934 f l f f f f 1 Patented July 14, 1936 UNITEDy STAT GFFQE PIEZOELECTRIC ELEMENT of Delaware Application June 9, 1934, Serial No. 729,931

7 Claims.

My invention relates to improvements in piezoelectric elements and methods of producing the same.

O ne of the objects of my invention is toprovide an improved method of producing piezoelectric elements in which the rate of growth of the seed crystal in the mother liquor is substantially greater than has been the case when methods proposed heretofore are followed.

Another object of my invention is toprovide an improved method of producing a piezo-electric element by which the possible strength of the electric field is substantially greater than has been the case with such elements constructed heretofore.

Another object is to provide an improved piezo-electric element which has advantages over those constructed heretofore in the Way of substantiallyless stiffness for flexural vibrations.

Another object is to provide an improved method for producing piezo-electric crystals by which undesirable spurious or weed crystal growth is eliminated.

Other objects `and advantages will hereinafter appear.

In accordance with my invention, an electrode is supported in a mother liquor with a seed crystal disposed within the body of the electrode and exposedto the liquor on opposite sides of the electrode. Crystal growth is caused to take place from the seed as a nucleus and over the opposite sides of the electrode to produce a piezo-electric element with an electrode embedded therein.

Further, in accordance with my invention, a flat electrode in the form of a piece of perforated metal foil is supported in a mother liquor between two spaced plates, With a seed crystal disposed at the center of and extending through the foil. Flat triangular wedges are placed between the plates and are disposed and shaped to form a cross-shape space with the seed crystal at the center, so that as the growth takes place from the seed as a nucleus, a cross-shape piezo-electric element is produced having an electrode embedded therein.

My invention resides in the improved method and construction of the character hereinafter described and claimed.

For the purpose. of illustrating my invention, an embodiment thereof is shown in the drawing, in which Figure 1 is a fragmentary perspective view, showing the manner in which the crystal-growing apparatus is set up in accordance With my invention;

Fig. 2 is an enlarged, fragmentary, sectional View, showing the manner in which the crystalgrowing apparatus is assembled in carrying out my improved method, and the manner of growth of the piezo-electric element;

Fig. 3 is a fragmentary, sectional view taken from Fig. 1, showing the condition of the parts before crystal growth starts; and

Fig. 4 is a simplified diagrammatic View, showing the complete crystal-growing apparatus capable of simultaneously producing a large number of the piezo-electric elements.

In setting up my apparatus, four triangular plates lil are placed upon a plate i2 of glass or corrosion-proof metal. i@ are directed inwardly toward the center and embrace right angles, as indicated in the drawing. The plates Il? are spaced and arranged as shown, with their apexes removed from the center of the plate I2 to form a cross-shape space I4 in which crystal growth is allowed to take place. An electrode IE in the form of a perforated piece of aluminum foil is then placed upon the plates Ill, after which a second set of triangular plates The apeXes of the plates I8 similar in shape and arrangement to the plates 25 It] is placed upon the foil I6. A plate of glass or metal, similar to the bottom plate I2, is the-n placed upon the second set of triangular plates i8 to form a cross-shape space 22 of the same configuration as and directly above the crossshape space Il.

Prior to placing the plate 2li in position, a ilat rectangular seed crystal Eil of Rochelle salt 'is inserted into a center rectangular hole 26 in the foil I6, the seed having a snug t in the hole.

Satisfactory results have been obtained by making the thickness of the seed 2li such that it extends about three-fourths of the. distance between the plates l2 and 2Q. The seed, furthermore, is

oriented so that its Y and Z axes bisect the apexes 40 of the right angles of the triangular plates i and IS. In one case, the length of the seed was 1A, inch, the width was 1A, inch, and the thickness was 1/8 inch. The electrode I6 was of aluminum foil 0.5 mil. thick, supported midway between the plates i2 and 2), parallel to the 100 faces (a faces) of the seed crystal.

Another set of four triangular spacer plates similar to the plates I El and I3 is then placed upon the plate 20, another foil electrode similar to the electrode I6 is placed upon this set of plates as before, a seed crystal is inserted into the central opening in this electrode, and a set of four triangular spacer plates is placed upon the foil and arranged in the same manner as shown in Fig. 1. A plate which would correspond to the plate 20 in Fig. 3 is then placed upon the top spacer plates. In this way the apparatus is built up to form a stack 28 with a plurality of compartments for growing as many crystals.

As shown in Fig. 1, the various plates and the foil electrodes are provided with a number of openings for receiving bolts 3i? which operate to properly align and hold the parts in position. When the apparatus has been built up to the desired height, nuts 32 are screwed upon the protruding ends of the bolts to clamp the parts together.

The seed crystals may be grown according to the method disclosed in Patent 1,746,144, of February 4, 1930 to J. I-I. Christopher.

The stack 28 of plates is placed into a glass or rust-proof metal tank 35;, and Rochelle salt solution of specific gravity 1.404 at 39 C. is poured into this tank very slowly. The tank @t is then closed and made air-tight to avoid evaporation by a gasket 36 of sponge rubber and a glass cover 38. 'Ihe gasket of sponge rubber is secured to the glass with waX.

The temperature of the solution or mother liquor 4S is reduced slowly over a period of about seven days until the total drop is about 3 C. Satisfactory results have been obtained by reducing the temperature about .1 C. per day for the first two days of growth, reducing the temperature .2 C. during the next day, and reducing the temperature about .5 C. thereafter.

For the purpose of obtaining the proper degrees and variations of temperature, the tank 34 is placed within an outer tank i2 having double walls between which heat insulating material is placed. The source of heat is shown as being in the form of an electric light bulb lli or other resistance heater connected to a supply 46 through a relay switch 48. The relay is connected to a supply 5i? through a thermostatic element 52 which functions to open the switch 43 if the temperature of the air within the tank i2 rises above a set value, and to close this switch if the temperature of the air drops below this value. The temperature at which the air within the tank 2 is maintained by the automatic action within the element 52 may be varied by adjustment of the rheostat 54. A fan 5t connected across the supply and disposed as shown within the tank 32 keeps the air within the tank in circulation. The temperature throughout the interior of the tank 42 is, therefore, substantially uniform.

As the temperature of the mother liquor is decreased in the manner explained, crystal growth takes place from the seed crystals as nucleu, and through the perforations in the foil sheets and over both sides of each sheet. This growth takes place only within the cross-shaped spaces, as determined by the rectangular spacer plates such as i8 and IS. Each of the piezo-electric elements grown in this way therefore, is cross-shaped, with the long edges at 45 with the Y and Z crystallographic aXes. In the use of these elements, an electric eld at right angles to the 100 faces causes bending in opposite sense in adjacent arms of the cross.

When used as an electro-mechanical transducer, the crystal requires an additional set of tin or aluminum foil electrodes fastened to the 100 faces with balsam dissolved in Xylol and connected together electrically, but insulated from the inner electrode.

For the purpose of eliminating the growth of spurious or weed crystals during the growing of the piezo-electric crystals, I propose to apply heat at the base of the tank 3ft during the growing period to produce at least some degree of circulation of the mother liquor and consequent temperature equalization thereof. To this end, a heating element 58, disposed underneath the tank 3Q, is connected across the supply 5G through a rheostat G by which the heating action can be controlled. The desired action, to prevent the growth of weed crystals, has been obtained by adjusting the rheostat 5@ so that the temperature of the solution nl@ is maintained about 1.3" C. above the temperature of the air in the outer tank t2.

Instead of slowly decreasing the temperature of the mother liquor during the growing operation, this temperature may be kept constant. Satisfactory results have been obtained by using a solution of Rochelle salt having a specic grayity of 1.404 at 39.5@ C., and using about 1000 c. c. of the solution available to feed each growing crystal if the temperature of the supersaturated solution is to be.` kept constant at about 1.7o C. below the initial saturation temperature during seven days of growth.

Fine wire screen of nickel or non-corroding metal can be used instead of the perforated foil.

In setting up the apparatus, it is proposed to tilt the same by placing wedges under the tank 612. In this way any air between the plates; is eliminated. These wedges are removed after the cover 33 is put on the tank 34.

From the foregoing it will be seen that in producing piezo-electric elements in accordance with my invention, crystal growth is caused to take place from the seed crystal as a nucleus and over the opposite sides of the electrode to produce a crystal with an electrode embedded therein. These elements are particularly adaptable to electro-acoustical devices where great compliance is required for production of low frequencies.

It will be understood that various modifications, within the conception of those skilled in the artl are possible without departing from the spirit of my invention or the scope of the claims.

I claim as my invention:

1. The method of producing a piezo-electric element which comprises supporting a hat electrode in a mother liquor and with a flat rectangular seed crystal disposed within the body of the electrode and exposed to the liquor on opposite sides of the electrode and with its a faces substantially parallel to the respective opposite faces of the electrode and oriented so that its Y and Z axes bisect respectively four right angles in the; plane of said axes and disposed about said seed with their apexes directed inwardly and removed from the respective sides of said seed to define a cross-shape region with said seed at the center thereof, and causing crystal growth from the seed as a nucleus and over the opposite sides of the electrode and only in the cross-shape region to produce a cross-,shape piezo-electric element with an electrode embedded therein.

2. The method of producing a piezo-electric element which comprises supporting a seed crystal in a mother liquor contained in a tank sealed substantially air-tight and causing crystal growth from the seed as a nucleus, and applying heat at the base of the tank during the growing period to produce at least some degree of circulation of the mother liquor and consequent temperature equalization thereof.

3. A cross-shape piezo-electric element whose long edges areI substantially at forty-five degrees with the Y and Z crystallographic axes.

4. The method of producing a piezo-electric element Which comprises supporting an electrode in a mother liquor and with a seed crystal disposed Within the body of the electrode and exposed to the liquor on opposite faces of the electrode, and causing crystal growth from the seed as a nucleus over the opposite faces of the` electrode While limiting the lateral growth of the seed to produce a piezo-electric element of a desired conguration having an electrode embedded therein.

5. The method of producing a piezo-electric element of improved ilexbility which comprises supporting a flat electrode in a mother liquor and with a at seed crystal disposed Within the body of the electrode and exposed to the liquor on opposite sides of the electrode and with its surfaces substantially parallel to the respective opposite faces of the electrode, and causing crystal growth from the seed as a nucleus over the opposite faces of the electrodes while directing the lateral growth of the seed in a cross-shape pattern.

6. A piezo-electric element having a cross-shape conguration.

'7. A cross-shape piezo-electric element with an electrode embedded therein.

SIDNEY BLOOMENTHAL. 

